sad vs melancholy

sad is not a noun while melancholy is a noun.

Word	Noun	Adjective	Verb	Adverb
sad	No	Yes	No	No
melancholy	Yes	Yes	No	No

sad (noun)	melancholy (noun)
	a humor that was once believed to be secreted by the kidneys or spleen and to cause sadness and melancholy
	a feeling of thoughtful sadness
	a constitutional tendency to be gloomy and depressed

sad (adjective)	melancholy (adjective)
experiencing or showing sorrow or unhappiness	grave or even gloomy in character
bad; unfortunate	characterized by or causing or expressing sadness
of things that make you feel sad